The invention relates to a high-temperature fuel cell with a fuel-gas chamber disposed between a bipolar plate and an anode of an electrolyte/electrode unit.
It is known that when water is electrolyzed the electrical current breaks down the water molecules to hydrogen (H2) and oxygen (O2) A fuel cell reverses this procedure. Electrochemical combination of hydrogen (H2) and oxygen (O2) to give water is a very effective generator of electric current. This occurs without any emission of pollutants or carbon dioxide if the fuel gas used is pure hydrogen (H2). Even with an industrial fuel gas, such as natural gas or coal gas, and with air (which may also have been enriched with oxygen (O2)) instead of pure oxygen (O2), a fuel cell produces markedly lower levels of pollutants and less carbon dioxide than other energy generators in which the energy is introduced from different sources. The fuel cell principle has been implemented industrially in various ways, and indeed with various types of electrolyte and with operating temperatures of from 80xc2x0 C. to 1000xc2x0 C.
Depending on their operating temperature, fuel cells are divided into low-, medium-, and high-temperature fuel cells, and these in turn have a variety of technical configurations.
In the case of a high-temperature fuel cell stack composed of a large number of high-temperature fuel cells, there is an upper bipolar plate, which covers the high-temperature fuel cell stack, and under the plate there are, in order, at least one contact layer, an electrolyte/electrode unit, a further contact layer, a further bipolar plate, etc.
The electrolyte/electrode unit here contains two electrodesxe2x80x94an anode and a cathodexe2x80x94and a solid electrolyte configured as a membrane disposed between the anode and the cathode. Each electrolyte/electrode unit here situated between two adjacent bipolar plates forms, with the contact layers situated immediately adjacent to the electrolyte/electrode unit on both sides, a high-temperature fuel cell, which also includes those sides of each of the two bipolar plates which are situated on the contact layers. This type of fuel cell, and others types, are known from the reference titled xe2x80x9cFuel Cell Handbookxe2x80x9d by A. J. Appleby and F. R. Foulkes, 1989, pp. 440-454, for example.
A single high-temperature fuel cell provides an operating voltage of less than one volt. A high-temperature fuel cell stack is composed of a large number of high-temperature fuel cells. The connection in series of a large number of adjacent high-temperature fuel cells can give an operating voltage of some hundreds of volts from a fuel-cell system. Since the current provided by a high-temperature fuel cell is highxe2x80x94up to 1,000 amperes in the case of large high-temperature fuel cellsxe2x80x94the electrical connection between the individual cells should preferably be one that gives rise to particularly low series electrical resistance under the above-mentioned conditions.
The electrical connection between two high-temperature fuel cells is provided by a bipolar plate, via which the anode of one high-temperature fuel cell is connected to the cathode of the other high-temperature fuel cell. The bipolar plate therefore has an electrical connection to the anode of one high-temperature fuel cell and to the cathode of the other high-temperature fuel cell. The electrical connection between the anode and the bipolar plate is provided by an electrical conductor, which may take the form of a nickel grid (see, for example, German Patent DE 196 49 457 C1). It has been found that the series electrical resistance between the electrical conductor and the bipolar plate is high. This has a serious adverse effect on the electrical performance of the high-temperature fuel cell stack.
It is accordingly an object of the invention to provide a high-temperature fuel cell which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which avoids any relatively high series electrical resistance even when used at high temperatures and to ensure a high conductivity even over prolonged periods.
With the foregoing and other objects in view there is provided, in accordance with the invention, a high-temperature fuel cell. The fuel cell contains a bipolar plate, an electrolyte/electrode unit having an anode, and a fuel-gas chamber formed between the bipolar plate and the anode. An oxidation buffer containing iron is disposed in the fuel-gas chamber.
The object is achieved by a high-temperature fuel cell of the type described in the introduction in which, according to the invention, the oxidation buffer is disposed in the fuel-gas chamber.
Experiments using a high-temperature fuel cell stack have shown that the electrical resistance between the electrical conductor and a bipolar plate formed Cr5Fe1Y2O3 rises, even after a short operating time at normal operating temperatures of between 850xc2x0 C. and 950xc2x0 C. The designation Cr5Fe1Y2O3 represents a chromium alloy that contains 5% by weight of Fe and 1% by weight of Y2O3. The increase in the electrical resistance is caused by an oxide layer which contains chromium oxide and even after a short operating period forms on the surface of that side of the bipolar plate which faces that chamber of the high-temperature fuel cell which carries fuel gas, known as the fuel-gas chamber for short. It also forms where the electrical conductor, for example the nickel grid, rests on the bipolar plate or is connected to the bipolar plate, for example by a weld spot or a soldering point. If the electrical conductor, for example the nickel grid, is spot-welded to the bipolar plate, during operation the oxide, amazingly, even migrates under these contact points, which are in the form of weld spots. Chromium oxide has higher electrical resistance than the unoxidized metals of the bipolar plate. Therefore, there is an oxide layer of poor conductivity between the electrical conductor and the bipolar plate, and this has an adverse effect on the series resistance of series-connected high-temperature fuel cells. The formation of chromium oxide takes place even at oxygen partial pressures of less than 10xe2x88x9218 bar. These oxygen partial pressures are generally also present in the fuel-gas chamber while the high-temperature fuel cell is operating.
The invention is based on the idea that a relatively high series electrical resistance is avoided and a high conductivity is ensured even over relatively long periods if the formation of the oxide layer on the bipolar plate is suppressed. This is achieved by the fact that the oxygen that is situated in the fuel-gas chamber of the high-temperature fuel cell while the latter is operating is taken up and stored by the oxidation buffer. Consequently, oxygen is withdrawn from the fuel gas and is then no longer available for oxidation of the bipolar plate. For this purpose, the oxidation buffer is configured in such a way that it takes up oxygen from the environment.
The term oxidation buffer is understood as meaning a device that takes up oxygen from the fuel gas during a with-load phase operation of the high-temperature fuel cell and releases the oxygen again during an idling phase of operation. In this way, the oxidation buffer is regenerated for a further with-load operating phase. The term with-load operation refers to the operating mode of the high-temperature fuel cell in which the high-temperature fuel cell generates significant amounts of electric current by the electrochemical combination of fuel gas and oxygen. The electrochemical combination does not take place during an idling mode. The idling mode is introduced, for example, by disconnecting the high-temperature fuel cells from a consumer.
In this context, an oxygen storage that takes up oxygen but cannot be regenerated again while the high-temperature fuel cell is operating is not considered to be an oxidation buffer. The property of being capable of regeneration, i.e. the possibility of removing the stored oxygen from the oxidation buffer again while the high-temperature fuel cell is operating represents a basic requirement of the oxidation buffer.
The appropriate material selected for the oxidation buffer is determined by the temperature and the oxygen partial pressure at which the high-temperature fuel cell is operated in the load phase and in the idling phase. A suitable material for the oxidation buffer is a material that during with-load operation is oxidized and during idling mode is reduced at the operating temperatures of the high-temperature fuel cell. The data required to select the material can be taken from tables. An example that can be mentioned in this context is the article by F. D. Richardson and J. H. E. Jeffes in J. Iron Steel Inst. 160, 261 (1948).
The invention ensures that the oxidation of the bipolar plate is suppressed as far as possible. This prevents a relatively high series electrical resistance of the high-temperature fuel cell and ensures a high conductivity even over prolonged periods.
The oxidation buffer expediently contains metallic iron, cobalt or nickel or an oxide of these metals. Iron or iron oxide is a particularly suitable material for the oxidation buffer at an operating temperature of around 900xc2x0 C. The oxygen partial pressure in the fuel-gas chamber during with-load operation is usually around 10xe2x88x9215 bar and in the idling mode is usually around 10xe2x88x9219 bar. Iron is a material that takes up oxygen at the above temperatures and oxygen partial pressures of around 10xe2x88x9215 bar and releases it at oxygen partial pressures of around 10xe2x88x9219 bar. Therefore, iron is a particularly suitable material for the oxidation buffer in these operating ranges.
At a lower operating temperature of a high-temperature fuel cell, for example around 850xc2x0 C., Co is recommended. Ni is suitable at around 800xc2x0 C. Further metals result from other operating temperatures.
An oxidation buffer that contains, for example, iron or iron oxide takes up oxygen from the fuel-gas chamber while the high-temperature fuel cell is operating with load and stores it by oxidation of the metallic iron. Regeneration of the oxidation buffer, i.e. reduction of the iron oxide, takes place during the idling phase of the high-temperature fuel cell, during which the oxygen partial pressure falls to around 10xe2x88x9219 bar. During regeneration of the oxidation buffer, the oxygen which has until then been bound in the iron oxide is liberated again and enters the fuel-gas chamber of the high-temperature fuel cell. So that in this phase there is little or no oxidation of the bipolar plate taking place, it is expedient to reduce the temperature of the high-temperature fuel cell during the regeneration phase, for example to around 850xc2x0 C. or below. Furthermore, it is appropriate to accelerate the regeneration of the oxidation buffer as far as possible by reducing the oxygen partial pressure. In an advantageous configuration of the invention, the oxidation buffer has a porous surface. A high porosity of the surface results in the oxidation buffer having a large surface area. The larger the surface area, the more oxygen can be taken up by oxidation per unit time by the oxidation buffer or can be released by reduction. A large surface area of the oxidation buffer can be achieved particularly effectively by configuring the oxidation buffer as a sponge.
In a further configuration of the invention, the oxidation buffer is configured as a foil. A foil, which may, for example, be laid as a layer onto a component of the high-temperature fuel cell, does not impair or scarcely impairs the flow of the fuel gas through the fuel-gas chamber. Furthermore, a foil is simple and inexpensive to produce.
In an advantageous configuration of the invention, the oxidation buffer is applied to the bipolar plate as a layer. This layer may be in the form of platelets that are attached to the bipolar plate, for example by welding. The layer may also be applied to the bipolar plate by electrode position, by thermal spraying or by a physical vapor deposition (PVD) process, such as sputtering, electron-beam vaporization or laser-beam vaporization. In this configuration of the invention, the layer is oxidized on the side that is remote from the bipolar plate while the high-temperature fuel cell is operating with load.
When using iron or iron oxide as the oxidation buffer, operation of the high-temperature fuel cell is switched over to the idling mode before the oxygen can reach the bipolar plate, i.e. before the layer is completely oxidized. As a result, the iron oxide of the oxidation buffer, which is formed during operation with load, is reduced to form metallic iron and the layer is regenerated.
The electrical connection between the anode and the bipolar plate of a high-temperature fuel cell is usually produced by a wire netting (see for example German Patent DE 196 49 457 C1).
In a preferred embodiment of the invention, a wire netting of this type in the fuel-gas chamber simultaneously serves as the oxidation buffer. This embodiment of the invention results in that there is no need to introduce an additional component into the fuel-gas chamber of the high-temperature fuel cell. It is appropriate for the wire netting to be configured as the oxidation buffer in such a way that the oxidation buffer is not situated in the immediate vicinity of the bipolar plate. Consequently, during regeneration of the oxidation buffer the oxygen that is released from the oxidation buffer is prevented as far as possible from passing into the bipolar plate.
In a further advantageous configuration of the invention, the bipolar plate is formed of Cr5Fe1Y2O3, i.e. contains 94% by weight of chromium, 5% by weight of Fe and 1% by weight of Y2O3. A bipolar plate of this type has proven to be suitable for operation in a high-temperature fuel cell in numerous tests.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a high-temperature fuel cell, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.